The present invention relates to the modulation of optical signals, and more particularly, to an optical modulation system which includes a self-damped electromechanical conformal grating.
High-speed optical data modulation systems are used for various applications including optical data storage and communications. These systems require data throughput in the megahertz frequency range. Substantial progress has been made in the development and implementation of microelectro-mechanical (MEMS) light modulators that operate efficiently at these frequencies. MEMS light modulators with a variety of designs have been used in applications such as display, optical processing, printing, optical data storage and spectroscopy. These light modulators produce spatial variations in the phase and/or amplitude of an incident light beam using arrays of individually addressable elements. For example, high-speed reflective phase gratings have been fabricated using suspended micromechanical ribbon elements, as described in U.S. Pat. No. 5,311,360 to Bloom et al. This device, also known as a grating light valve (GLV), can be fabricated with CMOS-like processes on silicon. Bloom et al. described a similar device in U.S. Pat. No. 5,459,610, with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. Bloom et al in U.S. Pat. No. 5,677,783 also presented a method for fabricating the device. Additional improvements in the design and fabrication of the GLV were described in U.S. Pat. No. 5,841,579 to Bloom et al. and in U.S. Pat. No. 5,661,592 to Bornstein et al.
A completely different class of electromechanical grating devices may be obtained by defining a grating structure in the supports below elongated ribbon elements, as disclosed in commonly-assigned U.S. Ser. No. 09/491,354 filed Jan. 26, 2000 entitled xe2x80x9cSpatial Light Modulator With Conformal Grating Elementsxe2x80x9d by Mark W. Kowarz. These devices, which are referred as electromechanical conformal gratings, function on the principle of a hidden grating. In the unactuated state, the grating structure is completely hidden from view and the device functions as a mirror. In the actuated state, the elongated ribbon elements deform to reveal the grating structure of the supports, thus generating a partially conformal diffraction grating. The operation of a electromechanical conformal grating is based on an attractive electrostatic force, which is produced by a voltage difference between a ground plane and the conducting layer on elongated ribbon elements. This attractive force changes the heights of the deformable portions of the elongated ribbon elements relative to the substrate. Modulation of the diffracted optical beam is obtained by appropriate choice of the voltage waveform. The voltage needed to deform a ribbon a certain distance depends on several factors including the tensile stress in the ribbon element, the length of the deformable portions of the ribbon and the distance between the ground plane and the top conductive layer.
The resonant frequency of the deformable portions of the elongated ribbon elements depends primarily on their tensile stress, density, and length. When a ribbon is actuated or released, the deformable portions ring at their resonant frequency, which is typically between 1 and 15 MHz. The mechanical response of the deformable portions of the elongated ribbon elements is damped by the flow and compression of the layer of gas beneath the ribbons. This phenomenon is referred to as squeeze film damping. It depends on the type of gas present, the pressure, film thickness etc. This damping determines the width of the resonant peak associated with the resonant frequency of the ribbons. As a result of this resonant ringing, the maximum frequency at which an electromechanical conformal grating can be operated is limited, and the diffracted light intensity contains undesirable temporal variations. These temporal variations in a data stream give rise to undesired data errors. Therefore, there is a need for an electromechanical conformal grating having increased operating speed and reduced temporal light intensity variations.
It is an object of the present invention to provide an optical modulation system with a self-damped electromechanical conformal grating for a beam of light in accordance with an input data stream that is particularly suitable for input data rates greater than 2 MHz.
This object is achieved by a system for modulating a beam of light in accordance with an input data stream having a data rate greater than 2 MHz, comprising:
(a) a source of light for directing light along a predetermined path;
(b) a self-damped electromechanical conformal grating disposed in the predetermined path, the self-damped electromechanical conformal grating including:
(i) an elongated ribbon element including a light reflective surface,
(ii) a substrate and a pair of end supports for supporting the elongated ribbon element at both ends over the substrate; and
(iii) at least one intermediate support between the end supports so that there are deformable portions of the elongated ribbon element above and movable into a channel containing a gas; and
(c) means responsive to the input data stream for applying forces to the an elongated ribbon element to cause the deformable portions of the elongated ribbon element to move into the channel so that the deformable portions of the elongated ribbon element are movable between first and second positions in accordance with the input data stream; and
(d) the self-damped electromechanical conformal grating modulating the light beam and directing the modulated light to a light utilization device where the modulated light can be recorded or decoded, the deformable portions of the elongated ribbon element being sufficiently damped to minimize the introduction of data errors into the modulated light beam.
In accordance with the present invention an optical data modulation system with a self-damped electromechanical conformal grating suitable for 2 MHz data rates is disclosed. The system represents a significant improvement over existing technology in terms of its data throughput, reliability, and manufacturability. The modulator system can readily be optimized at standard ambient conditions which substantially simplifies fabrication and packaging, and reduces per unit costs.